CODEWITHSUNDEEP
javascriptazurestored-proceduresazure-cosmosdbetag
Ruben Bartelink
Ruben Bartelink

Reputation: 61795

Azure CosmosDb Stored Procedure IfMatch Predicate

In a DocDb stored procedure, as the first step in a process retrieving data that I'm mutating, I read and then use the data iff it matches the etag like so:

collection.readDocument(reqSelf, function(err, doc) {
    if (doc._etag == requestEtag) {
        // Success - want to update
    } else {
        // CURRENTLY: Discard the read result I just paid lots of RUs to read 
        // IDEALLY: check whether response `options` or similar indicates retrieval
                    was skipped due to doc not being present with that etag anymore 
        ...
        // ... Continue with an alternate strategy
    }
});

Is there a way to pass an options to the readDocument call such that the callback will be informed "It's changed so we didn't get it, as you requested" ?

(My real problem here is that I can't find any documentation other than the readDocument undocumentation in the js-server docs)

Upvotes: 1

Views: 1115

Answers (2)

Nick Chapsas
Nick Chapsas

Reputation: 7200

Technically you can do that by creating a responseOptions object and passing it to the call.

function sample(selfLink, requestEtag) {
    var collection = getContext().getCollection();

    var responseOptions = { accessCondition: { type: "IfMatch", condition: requestEtag } };

    var isAccepted = collection.readDocument(selfLink, responseOptions, function(err, doc, options) {
        if(err){
            throw new Error('Error thrown. Check the status code for PreconditionFailed errors');
        }

        var response = getContext().getResponse();
        response.setBody(doc);

    });

    if (!isAccepted) throw new Error('The query was not accepted by the server.');
}

However, even if the etag you provide is not the one that the document has, you won't get an error and you will properly get the document itself back. It's just not supposed to work with that using the readDocument function in a stored procedure.

Upvotes: 1

Ruben Bartelink
Ruben Bartelink

Reputation: 61795

Thanks to some pushing from @Nick Chapsas, and this self-answer from @Redman I worked out that in my case I can achieve my goal (either read the current document via the self-link, or the newer one that has replaced it bearing the same id) by instead generating an Alt link within the stored procedure like so:

var docId = collection.getAltLink() + "/docs/"+req.id;
var isAccepted = collection.readDocument(docId, {}, function (err, doc, options) {
    if (err) throw err;
    // Will be null or not depending on whether it exists
    executeUpsert(doc);
});
if (!isAccepted) throw new Error("readDocument not Accepted");

Upvotes: 0

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